alt-text

The alt-text element is allowed inside either
startflag or endflag to provide alternate text
for an image when the imageref attribute sets an image to be used for flagging.
When no alternate text is specified for a revision flag, the default
alternate text for revprop start of change is a localized
translation of "Start of change", and the default alternate text for
revprop end of change is a localized translation of
"End of change".